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Handle detour patches across page boundaries #1535

Merged
merged 3 commits into from
Jul 17, 2021
Merged

Handle detour patches across page boundaries #1535

merged 3 commits into from
Jul 17, 2021

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asherkin
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On Linux if a detour crossed a page boundary we would only change the
memory protection of the first page (as we were aligning the address as
required, but not taking into account the length).

I don't have an easy way to test this but it looks correct. addr + len
doesn't appear to need to be aligned though, so another option could be
to use (addr - startPage) + length as len.

Also fixed a non-zero offset being passed into CDetour's ApplyPatch
function - this is never done internally anywhere, but it doesn't hurt
to fix it.

Fixes #984

On Linux if a detour crossed a page boundary we would only change the
memory protection of the first page (as we were aligning the address as
required, but not taking into account the length).

I don't have an easy way to test this but it looks correct. `addr + len`
doesn't appear to need to be aligned though, so another option could be
to use `(addr - startPage) + length` as len.

Also fixed a non-zero offset being passed into CDetour's ApplyPatch
function - this is never done internally anywhere, but it doesn't hurt
to fix it.

Fixes #984
@asherkin asherkin requested a review from dvander July 17, 2021 17:53
mprotect(addr2, sysconf(_SC_PAGESIZE), prot);
long pageSize = sysconf(_SC_PAGESIZE);
void *startPage = ke::AlignedBase(addr, pageSize);
void *endPage = ke::AlignedBase((intptr_t)addr + length, pageSize);
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Should this be addr + length - 1 if the sequence runs right up to the page boundary?

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I did thing that initially but I think this is correct, if you imagine a zero-length sequence doing -1 would take us one page back, and a sequence equal to the length of a page seems like it'll do the right thing here as well (endPage would be the same as startPage, but one more byte would take it to the next page).

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startPage = AlignedBase(0, 4096) = 0
endPage = AlignedBase(0 + 4096, 4096) = 4096
len = endPage - startPage + pageSize = 4096 - 0 + 4096 = 8192

So I think you'd wind up with two pages? That said, it's pretty edge casey and doesn't really matter.

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Ah, yes, that does make sense! Yeah, I think I'd prefer going over in that edge case than trying to special-case zero length.

The code here can only use a maximum length of 20 currently :D

mprotect(addr2, sysconf(_SC_PAGESIZE), prot);
long pageSize = sysconf(_SC_PAGESIZE);
void *startPage = ke::AlignedBase(addr, pageSize);
void *endPage = ke::AlignedBase((intptr_t)addr + length, pageSize);
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startPage = AlignedBase(0, 4096) = 0
endPage = AlignedBase(0 + 4096, 4096) = 4096
len = endPage - startPage + pageSize = 4096 - 0 + 4096 = 8192

So I think you'd wind up with two pages? That said, it's pretty edge casey and doesn't really matter.

@asherkin asherkin merged commit 5b7c9c5 into master Jul 17, 2021
@asherkin asherkin deleted the bug-984 branch July 17, 2021 19:53
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ProtectMemory called with wrong address in public/CDetour/detourhelpers.h
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